1. Field of the Invention
The present invention generally relates to image processing utilizing incoherent radiation and is specifically directed to image processing with holographic spatial filters which, when combined with a lens in an optical system, provide a priori specified transfer functions.
2. Discussion of the Prior Art
The processing of images containing graphic information with spatially incoherent electromagnetic radiation has a distinct advantage over systems utilizing coherent radiation. Much graphic information is printed on paper. In order to process images containing such graphic information with coherent laser-optical systems, the information must be converted into transparency form. With spatially incoherent radiation, graphic images may be processed directly from the paper.
The use of a hologram for pattern detection by correlation filtering in spatially incoherent optical systems was suggested in 1965 by Armitage and Lohmann in Appl. Opts. 4, 461 (1965). This system, including an incoherent light source and a combination of a lens and a spatial filter, is insensitive to shifts in the position of the hologram. Armitage and Lohmann recognized that it is not an easy problem to prescribe first the optical transfer function for the system to be provided by the spatial filter and then look for a suitable pupil function for defining the hologram. The pattern detection application, which utilizes a matched filter initially formed from an image of the pattern to be detected, and which finds utility in shift invarient character recognition, has appeared to be the only exception to this difficult mathematical problem.
Lowenthal and Werts, in Compt. rend. 266 B, 542 (1968), noted that both correlation and convolution image processing operations could be performed with an optical system including a hologram and lens using spatially incoherent light. In particular, they state that when one is given, a priori, the point spread function for an optical system, the required pupil function for the lens may be obtained by recording this point spread function in a hologram. Lowenthal and Werts' only reported application, however, was to employ the correlation operation to recognize alphabetic characters.
In 1971, Maloney, in Appl. Opt. 10, 2127 (1971) and Appl. Opt. 10, 2554 (1971), proposed a system for character recognition using Lohmann's incoherent correlation techniques. He incorporated the lens directly into the hologram using methods well known in the art.
Also in 1971, Lohmann and Werlich, in Appl. Opt. 10, 670 (1971), proposed further applications utilizing pattern detection by correlation filtering with spatially incoherent optical systems. Correlations between line drawings and between grey-tone objects were disclosed. However, Lohmann and Werlich stated: "Additional spatial filtering for better target discrimination (such as suppression of the useless low frequencies) cannot be done in incoherent image formation, at least not as straight-forwardly as in the coherent method." Lohmann and Werlich then go on to suggest the use of computer generated Fourier holograms as correlation filtering elements for pattern detection.
Thus, the prior art fails to disclose or suggest how to proceed from specific incoherent radiation correlation techniques to the more general optical processing applications which require an a priori specification of the transfer function.